Hearing loss, which may be due to many different causes, is generally of two types, conductive and sensorineural. Sensorineural hearing loss occurs when there is damage to the inner ear, or to the nerve pathways connecting the inner ear to the brain. Conductive hearing loss occurs when the normal mechanical pathways that provide sound to the cochlea are impeded, for example, by damage to the ossicular chain or ear canal. However, individuals suffering from conductive hearing loss may retain some form of residual hearing because the hair cells in the cochlea may remain undamaged. As a result, individuals suffering from conductive hearing loss typically receive a hearing prosthesis that generates mechanical motion of the cochlea fluid. Still other individuals suffer from mixed hearing losses, that is, conductive hearing loss in conjunction with sensorineural hearing. Such individuals may have damage to the outer or middle ear, as well as to the inner ear (cochlea). Individuals suffering from conductive hearing loss typically receive an acoustic hearing aid. Unfortunately, not all individuals suffering from conductive hearing loss are able to derive suitable benefit from hearing aids.
Another type of hearing prosthesis is a cochlear implant. Cochlear implants provide electrical stimulation via, e.g. an electrode assembly with stimulating electrode contacts positioned as close as possible to the auditory nerve, essentially bypassing the cochlear hair cells. The application of a stimulation pattern to the nerve endings causes impulses to be sent to the brain via the auditory nerve, resulting in the brain perceiving the impulses as sound.
Insertion of a cochlear implant electrode assembly may cause trauma to the recipient's cochlea. For example, when a surgeon inserts an electrode assembly into the scala tympani, the basilar membrane may be bruised, punctured or torn. Such physical trauma may lead to a temporary or permanent change in the recipient's residual hearing characteristics.